Liposomes retention rate

Crowe and Crowe [3.39] proved that it is sufficient for certain liposomes, e. g. egg phosphatidyl-choline (DPPC), to be vitrified by trehalose or dextran during freezing and freeze drying. In trehalose the retention rate was almost 100 %, and in dextran more than 80 %. This did not apply to egg PC-liposomes Dextran as CPA alone led to an almost total loss of the CF-indicator, but addition of dextran into a trehalose solution (Fig. 3.20) also reduced the retention rate of CF substantially, e. g. from 90 % in a pure trehalose to approx. 45 % if trehalose and dextran were in equal amounts in the solution. Since T of dextran is approx. -10 °C and Tg- of trehalose is -30 to -32 °C, dextran should form a glass phase at much higher temperatures than trehalose. Therefore the stabilization of egg- PC with trehalose cannot be related with the vitrification. Crowe showd with IR spectroscopy that egg-PC freeze dried with 2 g trehalose/g lipid had almost the identical spectrographic characteristics as the hydrous lipid Trehalose molecules replaced the water molecules, and hydrogen... 

Fig. 3.21. Retention rate of liposome-encapsulated hemoglobin as a function of the time elapsed after the reconstitution of the freeze dried LEH, with different trehalose concentrations as CPA. 1, no trehalose 2, 10 mM 3, 50 mM 4, 150 mM 5, 300 mM trehalose (Fig. 2 from [3.43]).

To test and measure the retention rate of liposomes, carboxyfluorescein (CF) can be used. Ausborn and Nuhn [3.38] studied different lipid vesicles, e.g. egg lecithin (EPC), hydrated egg lecithin (HEPC), cholesterol (CHOL) and mixtures thereof. For... 

To freeze-dry liposomes requires stabilization not only during freezing but also during drying and storage of the dry producL Talsma [1.34, p. 106] showed that the retention rate of small liposomes (0.13 pm) was 24.1% and greater than that of large liposomes (0.28 pm), which had only a 7.4% retention rate. 

Amphotericin B encapsulated in charged liposomes demonstrated 10-fold higher transport rates across the skin compared with neutral liposomes [279], In a similar manner, the retention of acyclovir from positively charged vesicles was much higher compared with other formulations [277], This could be attributed to the attraction... 

As mentioned above, the ability to adsorb to the cornea and an optimal drug release rate have been defined as the two liposomal attributes most responsible for increasing bioavailability after topical ocular administration. A number of factors, including drug encapsulation efficiency, liposome size and charge, distribution of the drug within liposomes, stability of liposomes in the conjunctival sac and ocular tissues, their retention in the conjunctival sac, and most importantly their affinity toward the corneal surface and the rate of release of the encapsulated drug, have... 

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